MAGMATIC ACTIVITY AND STRATIGRAPHIC COMPLEXITIES OF THE WASP SPRING AND BURRO MESA RHYOLITES AND RELATED INTRUSIVE ROCKS WITHIN BIG BEND NATIONAL PARK, TEXAS USING 40Ar/39Ar GEOCHRONOLOGY

Intrusive and extrusive felsic magmatic activity was widespread in the Big Bend region since the Eocene. In Big Bend National Park (BBNP), the Wasp Spring Rhyolite (WSR) and Burro Mesa Rhyolite (BMR) members of the Burro Mesa Formation and related dikes and intrusions represent the youngest episodes of felsic activity in the western part of the park. 40Ar/39Ar data from the new geologic map of BBNP of Turner et al., 2011 along with new 40Ar/39Ar data is presented here. The WSR and BMR vary considerably in thickness and extent, as well as their relative stratigraphic positions. These rhyolites yield variable 40Ar/39Ar ages, but indicate that eruptions occurred from 29.5 through 28.9 Ma. Thick deposits of ash-flow tuffs north and east of Cerro Castellan were originally thought to be Mule Ear Spring Tuff (MEST-33.6 Ma) based on morphology and stratigraphic position, but 40Ar/39Ar ages from these units show that they are in fact Wasp Spring deposits. The lack of MEST and other Chisos tuffs suggest a lack of volcanic deposition and/or erosion occurred between ~33.7 Ma and 30.0 Ma. Based on the geochronology and detailed field mapping, the WPR consists of base surge and air-fall deposits, non-welded, welded, and densely-welded ash-flow tuffs and can resemble the MEST. The BMR is generally found stratigraphically above the WSR and deposits can exceed 200m in thickness. The BMR erupted after the WSR at about 29.4 to 28.9 Ma, but recent sanidine ages from a amphibole-rich and amphibole-poor BMR indicate it erupted in discrete episodes at 29.33 Ma and 28.98 Ma at Trap Mountain indicating that the BMR was erupting in cycles of ~350,000 ka at this location. Samples of BMR from other locations in the western part of the park range in age from 29.45 to 29.25 Ma, but additional geologic mapping and geochronology may reveal younger eruptive centers/lavas. Intrusive rocks are pervasive throughout BBNP. Several felsic intrusions were dated and suggest that a discrete felsic magmatic pulse occurred nearly concurrently with the uppermost basaltic lava flows above the BMR. A felsic intrusion containing sanidine and quartz was dated at 29.03 Ma at Burro Spring just west of the Burro Mesa Pouroff. A dike of similar composition was dated along the Ross Maxwell Scenic Drive north of Blue Creek Ranch at 29.09 Ma.